The present invention relates to novel fluid catalytic cracking catalysts comprising microspheres containing Y-faujasite zeolite and having exceptionally high activity and other desirable characteristics, methods for making such catalysts and the use of such catalysts for cracking petroleum feedstocks, particularly under short residence time processes.
Since the 1960's, most commercial fluid catalytic cracking catalysts have contained zeolites as an active component. Such catalysts have taken the form of small particles, called microspheres, containing both an active zeolite component and a non-zeolite component. Frequently, the non-zeolitic component is referred to as the matrix for the zeolitic component of the catalyst. The non-zeolitic component is known to perform a number of important functions, relating to both the catalytic and physical properties of the catalyst. Oblad described those functions as follows: “The matrix is said to act as a sink for sodium in the sieve thus adding stability to the zeolite particles in the matrix catalyst. The matrix serves the additional function of: diluting the zeolite; stabilizing it towards heat and steam and mechanical attrition; providing high porosity so that the zeolite can be used to its maximum capacity and regeneration can be made easy; and finally it provides the bulk properties that are important for heat transfer during regeneration and cracking and heat storage in large-scale catalytic cracking.” A. G. Oblad Molecular Sieve Cracking Catalysts, The Oil And Gas Journal, 70, 84 (Mar. 27, 1972).
In prior art fluid catalytic cracking catalysts, the active zeolitic component is incorporated into the microspheres of the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into microspheres in a separate step. In the second technique, the in-situ technique, microspheres are first formed and the zeolitic component is then crystallized in the microspheres themselves to provide microspheres containing both zeolitic and non-zeolitic components.
It has long been recognized that for a fluid catalytic cracking catalyst to be commercially successful, it must have commercially acceptable activity, selectivity, and stability characteristics. It must be sufficiently active to give economically attractive yields, it must have good selectivity towards producing products that are desired and not producing products that are not desired, and it must be sufficiently hydrothermally stable and attrition resistant to have a commercially useful life.
Generally, FCC is commercially practiced in a cyclic mode. During these operations, the hydrocarbon feedstock is contacted with hot, active, solid particulate catalyst without added hydrogen, for example, at pressures of up to about 50 psig and temperatures up to about 650.degree. C. The catalyst is a powder with particle sizes of about 20-200 microns in diameter and with an average size of approximately 60-100 microns. The powder is propelled upwardly through a riser reaction zone, fluidized and thoroughly mixed with the hydrocarbon feed. The hydrocarbon feed is cracked at the aforementioned high temperatures by the catalyst and separated into various hydrocarbon products. As the hydrocarbon feed is cracked in the presence of cracking catalyst to form gasoline and olefins, undesirable carbonaceous residue known as “coke” is deposited on the catalyst. The spent catalyst contains coke as well as metals that are present in the feedstock. Catalysts for FCC are typically large pore aluminosilicate compositions, including faujasite or zeolite Y.
The coked catalyst particles are separated from the cracked hydrocarbon products, and after stripping, are transferred into a regenerator where the coke is burned off to regenerate the catalyst. The regenerated catalyst then flows downwardly from the regenerator to the base of the riser.
These cycles of cracking and regeneration at high flow rates and temperatures have a tendency to physically break down the catalyst into even smaller particles called “fines”. These fines have a diameter of up to 20 microns as compared to the average diameter of the catalyst particle of about 60 to about 100 microns. In determining the unit retention of catalysts, and accordingly their cost efficiency, attrition resistance is a key parameter. While the initial size of the particles can be controlled by controlling the initial spray drying of the catalyst, if the attrition resistance is poor, the catalytic cracking unit may produce a large amount of the 0-20 micron fines which should not be released into the atmosphere. Commercial catalytic cracking units include cyclones and electrostatic precipitators to prevent fines from becoming airborne. Those skilled in the art also appreciate that excessive generation of catalyst fines increases the cost of catalyst to the refiner. Excess fines can cause increased addition of catalyst and dilution of catalytically viable particles.
U.S. Pat. No. 4,493,902, the teachings of which are incorporated herein by cross-reference, discloses novel fluid cracking catalysts comprising attrition-resistant, high zeolitic content, catalytically active microspheres containing more than about 40%, preferably 50-70% by weight Y faujasite and methods for making such catalysts by crystallizing more than about 40% sodium Y zeolite in porous microspheres composed of a mixture of two different forms of chemically reactive calcined clay, namely, metakaolin (kaolin calcined to undergo a strong endothermic reaction associated with dehydroxylation) and kaolin clay calcined under conditions more severe than those used to convert kaolin to metakaolin, i.e., kaolin clay calcined to undergo the characteristic kaolin exothermic reaction, sometimes referred to as the spinel form of calcined kaolin. In a preferred embodiment, the microspheres containing the two forms of calcined kaolin clay are immersed in an alkaline sodium silicate solution, which is heated, preferably until the maximum obtainable amount of Y faujasite is crystallized in the microspheres.
In practice of the '902 technology, the porous microspheres in which the zeolite is crystallized are preferably prepared by forming an aqueous slurry of powdered raw (hydrated) kaolin clay (Al2O3:2SiO2:2H2O) and powdered calcined kaolin clay that has undergone the exotherm together with a minor amount of sodium silicate which acts as fluidizing agent for the slurry that is charged to a spray dryer to form microspheres and then functions to provide physical integrity to the components of the spray dried microspheres. The spray dried microspheres containing a mixture of hydrated kaolin clay and kaolin calcined to undergo the exotherm are then calcined under controlled conditions, less severe than those required to cause kaolin to undergo the exotherm, in order to dehydrate the hydrated kaolin clay portion of the microspheres and to effect its conversion into metakaolin, this resulting in microspheres containing the desired mixture of metakaolin, kaolin calcined to undergo the exotherm and sodium silicate binder. In illustrative examples of the '902 patent, about equal weights of hydrated clay and spinel are present in the spray dryer feed and the resulting calcined microspheres contain somewhat more clay that has undergone the exotherm than metakaolin. The '902 patent teaches that the calcined microspheres comprise about 30-60% by weight metakaolin and about 40-70% by weight kaolin characterized through its characteristic exotherm. A less preferred method described in the patent, involves spray drying a slurry containing a mixture of kaolin clay previously calcined to metakaolin condition and kaolin calcined to undergo the exotherm but without including any hydrated kaolin in the slurry, thus providing microspheres containing both metakaolin and kaolin calcined to undergo the exotherm directly, without calcining to convert hydrated kaolin to metakaolin.
In carrying out the invention described in the '902 patent, the microspheres composed of kaolin calcined to undergo the exotherm and metakaolin are reacted with a caustic enriched sodium silicate solution in the presence of a crystallization initiator (seeds) to convert silica and alumina in the microspheres into synthetic sodium faujasite (zeolite Y). The microspheres are separated from the sodium silicate mother liquor, ion-exchanged with rare earth, ammonium ions or both to form rare earth or various known stabilized forms of catalysts. The technology of the '902 patent provides means for achieving a desirable and unique combination of high zeolite content associated with high activity, good selectivity and thermal stability, as well as attrition-resistance.
The aforementioned technology has met widespread commercial success. Because of the availability of high zeolite content microspheres which are also attrition-resistant, custom designed catalysts are now available to oil refineries with specific performance goals, such as improved activity and/or selectivity without incurring costly mechanical redesigns. A significant portion of the FCC catalysts presently supplied to domestic and foreign oil refiners is based on this technology. Refineries whose FCC units are limited by the maximum tolerable regenerator temperature or by air blower capacity seek selectivity improvements resulting in reductions in coke make while the gas compressor limitations make catalysts that reduce gas make highly desirable. Seemingly a small reduction in coke can represent a significant economic benefit to the operation of an FCC unit with air blower or regenerator temperature limitations.
The activity and selectivity characteristics of the catalysts formed by the process of the '902 patent are achieved even though, in general, the catalysts have relatively low total porosity as compared to fluid catalytic cracking catalysts prepared by incorporating the zeolite content into a matrix. In particular, the microspheres of such catalysts, in some cases, have a total porosity of less than about 0.15 cc/g. or even less than about 0.10 cc/g. In general, the microspheres of the '902 patent have a total porosity of less than 0.30 cc/g. As used herein, “total porosity” means the volume of pores having diameters in the range of 35-20,000 Å, as determined by the mercury porosimetry technique. The '902 patent noted that it was surprising that microspheres having a total porosity of less than about 0.15 cc/g. exhibit the activity and selectivity characteristics found. For example, such a result is contrary to the prior art disclosures that low pore volumes “can lead to selectivity losses due to diffusional restrictions.”
It is believed that the relatively low porosity of the catalyst microspheres formed as in the '902 patent does not adversely affect activity and selectivity characteristics, since the microspheres of the '902 patent are not diffusion limited relative to the typical FCC processing conditions which were used at the time of the patent. In particular, catalyst contact time with the feed to be cracked was typically 5 seconds or more. Thus, while typical FCC catalysts formed by mechanically incorporating the zeolite within a matrix may have been more porous, the reaction time in prior art FCC risers did not yield any advantage in activity or selectivity. This result inspired the conclusion that transport processes were not at all limiting in FCC catalysts, at least outside the zeolite structure. Assertions made to the contrary were inconsistent with the facts and easily dismissed as self-serving. Importantly, the attrition resistance of the microspheres prepared in accordance with the '902 patent was superior to the conventional FCC catalysts in which the crystallized zeolite catalytic component was physically incorporated into the non-zeolitic matrix.
Recently, however, FCC apparatus have been developed which drastically reduce the contact time between the catalyst and the feed which is to be cracked. Conventionally, the reactor is a riser in which the catalyst and hydrocarbon feed enter at the bottom of the riser and are transported through the riser. The hot catalyst effects cracking of the hydrocarbon during the passage through the riser and upon discharge from the riser, the cracked products are separated from the catalyst. The catalyst is then delivered to a regenerator where the coke is removed, thereby cleaning the catalyst and at the same time providing the necessary heat for the catalyst in the riser reactor. The newer riser reactors operate at lower residence time and higher operating temperatures to minimize coke selectivity and delta coke. Several of the designs do not even employ a riser, further reducing contact time to below one second. Gasoline and dry gas selectivity can improve as a result of the hardware changes. These FCC unit modifications are marketed as valuable independent of the type of catalyst purchased, implying an absence of systematic problems in state of the art catalyst technology.
The processing of increasingly heavier feeds in FCC type processes and the tendency of such feeds to elevate coke production and yield undesirable products have also led to new methods of contacting the feeds with catalyst. The methods of contacting FCC catalyst for very short contact periods have been of particular interest. Thus, short contact times of less than 3 seconds in the riser, and ultra short contact times of 1 second or less have shown improvements in selectivity to gasoline while decreasing coke and dry gas production.
To compensate for the continuing decline in catalyst to oil contact time in FCC processing, the “equilibrium” catalysts in use have tended to become more active. Thus, increases in the total surface area of the catalyst need to be achieved and as well, the level of rare earth oxide promoters added to the catalysts are increasing. Moreover, cracking temperatures are rising to compensate for the reduction in conversion. Unfortunately, it has been found that the API gravity of the bottoms formed during short contact time (SCT) often increases after a unit revamp, leading some to suggest that the heaviest portion of the hydrocarbon feed takes longer to crack. Further, while a high total surface area of the catalyst is valued, the FCC process still values attrition resistance. Accordingly, while not obvious to those participating in the art, it has become increasingly likely that an optimization of FCC catalysts for the new short contact time and ultra short contact time processing which is presently being used is needed.
It is now theorized, that under the short contact time processing of hydrocarbons, further improvements can be gained by eliminating diffusion limitations that may still exist in current catalysts. This is being concluded even as these materials excel at the application. It is theorized that improvements in these catalysts may be produced by optimization of catalyst porosity and the elimination of active site occlusion and diffusional restrictions of the binder phases present in catalysts prepared by the so-called incorporation method.
In commonly assigned U.S. Pat. No. 6,943,132, it has been found that if the non-zeolite, alumina-rich matrix of the catalyst is derived from an ultrafine hydrous kaolin source having a particulate size such that 90 wt. % of the hydrous kaolin particles are less than 2 microns, and which is pulverized and calcined through the exotherm, a macroporous zeolite microsphere can be produced.
The ultrafine hydrous kaolin is dried in a spray dryer, or suitable unit operation, then deagglomerated using high energy pulverizers, or dry milling procedures. This unit operation is practiced to reduce agglomerates and return calciner feed to a particle size similar to what was measured in a slurry as noted above. The presence of agglomerated structures alter the particle size and bulk density properties of the calcined kaolin. During phase change from hydrous kaolin, agglomeration and sintering occurs. The measured particle size coarsens throughout the particle size ranges. Large agglomerated structures have higher density thus lower porosity. Structuring prior to calcination expands the pore volume by cementing particle contact points, which in fully calcined kaolins are theoretically maintained by expelled amorphous silica. The thermal transition to spinel expels one mol of silica per mol of spinel formed. Mullite transition from spinel expels four additional mols.
More generally, the FCC catalyst matrix useful in this invention to achieve FCC catalyst macroporosity is derived from alumina sources, such as kaolin calcined through the exotherm, that have a specified water pore volume, which distinguishes over prior art calcined kaolin used to form the catalyst matrix. The water pore volume is derived from an Incipient Slurry Point (ISP) test, which is described in the patent.
The morphology of the microsphere catalysts which are formed according to U.S. Pat. No. 6,943,132 is unique relative to the in-situ microsphere catalysts formed previously. Use of a pulverized, ultrafine hydrous kaolin calcined through the exotherm yields in-situ zeolite microspheres having a macroporous structure in which the macropores of the structure are essentially coated or lined with zeolite subsequent to crystallization. Macroporosity as defined herein means the catalyst has a macropore volume in the pore range of 600-20,000 Å of at least 0.07 cc/gm mercury intrusion. The catalysts also have a BET surface area less than 500 m2/g. The novel catalyst is optimal for FCC processing, including the short contact time processing in which the hydrocarbon feed is contacted with a catalyst is for times of about 3 seconds or less.
High porosity within the microsphere is important to maximize catalytic activity by eliminating typical rate reductions due to diffusion of the crude oil molecules within the microsphere structure. As the porosity of a microsphere is increased, however, the rate at which the microsphere fractures and attrits into finer particles within the FCC unit operating environment increases; resulting in increased fresh catalyst addition rates and increased particulate emission from the unit. Processing or compositional mechanisms for reducing the rate at which an FCC catalyst attrits for a given total pore volume is of fundamental importance to improving the performance and corresponding value of the catalyst.